The present invention relates to a new process for the production of synthetic resin composite parts based on polyurethane by covering a polyurethane core material which has positioned inside a mold, with a reaction mixture which reacts to form a polyurethane, the reaction mixture being based on specific polyhydroxyl compounds to optimize the bond between the two polyurethane materials. Processes for the production of synthetic resin composite parts are known.
European patent 69,076 describes in very general terms a process by which a previously produced foam core is covered with a uniform covering layer by the pressure-free introduction of a foamable mixture into an open or closed mold. The exact positioning of the core is ensured by its buoyancy. The core preferably consists of polystyrene foam while the outer layer preferably consists of chemically expanded epoxide resin. No details are given concerning the bond between the core and the covering material.
U.S. Pat. No. 3,954,926 (corresponding to German Auslegeschrift 2,127,582) describes a process by which cellular composite parts having compact, fault-free surfaces are produced by the successive introduction of several starting substances into a closed mold which has an unchangeable cavity. In this process, the substances successively introduced comprise at least a first foamable synthetic resin and a second synthetic resin which may be foamable or not and forms the surface covering under compression of the first substance.
U.S. Pat. No. 4,539,166 (corresponding to German Offenlegungsschrift 3,219,039) describes a process in which two different reaction systems are successively introduced into a mold, an internal mold release agent being added to the second substance to improve the release of the product from the mold.
It was found that problems of adherence between the inner and the outer synthetic resin layer normally arise, not only in those systems in which internal mold release agents are used but in general. These problems also occur in multilayered moldings produced entirely from polyurethane materials.
It was therefore an object of the present invention to provide an improved process for the production of composite bodies based on polyurethanes, in which the covering layer, which is preferably a light fast polyurethane, adheres firmly to the polyurethane core without any problems arising in the removal from the mold.